Apparatus and method for checking the machining process of a machine tool

ABSTRACT

An apparatus and a method are provided for checking a machining process of a machine tool, for example a grinding machine, to obtain information about the quantity of a material removed and the wear condition of a grinding-wheel. The apparatus comprises an acoustic sensor adapted to detect acoustic signals generated by contact between the grinding-wheel and the piece being machined, and a force-detecting sensor adapted to detect the force applied between the grinding-wheel and the piece during the machining. The method comprises processing signals provided by both sensors, defining time intervals on the basis of the signals of the acoustic sensor, and monitoring the signals of the force-detecting sensor in the time intervals.

TECHNICAL FIELD

The present invention relates to an apparatus for checking the machiningprocess of a machine tool with a base and a tool-carrier slide, movablewith respect to the base and carrying a tool, the apparatus including asupport and reference system for supporting and referring a piece to bechecked, a first sensor, adapted for sensing the force occurring betweenthe tool and the piece and for emitting a first output signal and aprocessing and control unit connected to the first sensor and adaptedfor processing the first output signal.

The invention also relates to a method for checking the machiningprocess of a machine tool.

BACKGROUND ART

The ever-pressing requirements to increase the productivity of machinetools, to improve the quality of the mechanical pieces that thesemachine tools produce, on the basis of ever-tighter tolerances, and todiminish the production costs call for minimizing the machine down-timesand concurrently performing, whenever necessary, all the maintenanceoperations required for avoiding both the down-grade of the quality ofthe produced pieces and the costs deriving from scraps and re-machiningof pieces.

For checking the machining process of computer numerical control (“CNC”)machine tools, as lathes, grinding machines, milling machines, etc.there are utilized apparatuses provided with sensors that detect themagnitude of physical features connected to the process to be checkedand indicate, to the machine numerical control, or directly to theoperator, the need to perform maintenance or corrective procedures.

Similar apparatuses are those, for example, that detect the wear or thebreakage of the tools and signal the need for their substitution orsharpening.

U.S. Pat. No. 5,070,655 discloses an apparatus for monitoring somespecific working conditions of a machine tool, more particularly fordetecting grinding wheel sharpness, loss of coolant or excessivevibrations.

The apparatus processes signals indicative of the power consumption, asprovided by a power-detecting sensor, and signals indicative of themechanical vibrations, as provided by an accelerometer, and emits avisual signal (in the form of a green, a yellow or a red light, thelatter is associated with an acoustic signal) for indicating to theoperator the condition of the process under control.

U.S. Pat. No. 5,718,617 discloses a system for measuring a forceexisting between a grinding wheel and a piece in the course of themachining of a computer numerical control (“CNC”) grinding machine, by aforce-detecting sensor mounted between the ball screw that activates thetool-carrier slide and the machine base. In the machine there can alsobe mounted accelerometers for compensating the signals arriving from theforce-detecting sensor by removing components of said signals thatdepend on vibrations.

U.S. Pat. No. 5,044,125 discloses a machine tool, more specifically agrinding machine, with a force transducer that comprises piezoelectricsensors mounted adjacent the wheelhead to measure the magnitude of theforce occurring between the grinding wheel and the piece. The signalsoutput by the force transducer are sent to the numerical control thatdetermines when there is the need to perform a grinding wheel dressingcycle.

The signal output by the force transducer is suitably processed andprovides significant information relating to the machining or to thegrinding wheel sharpness, but does not enable to accurately distinguishthe time intervals when the grinding wheel is actually in contact withthe piece. In fact, the amplitude of the signal output by the forcetransducer slowly increases when contact between grinding wheel andpiece occurs and it slowly decreases when the grinding wheel is detachedfrom the surface of the piece, as hereinafter described. Furthermore,immediately after the displacing of the grinding wheel away from thepiece, the output signal of the force transducer has spuriouscomponents, due to the displacing of parts of the machine tool, that canhave an amplitude comparable with that of the signal in the course ofthe machining and must not be considered during the processing.

DISCLOSURE OF INVENTION

An object of the present invention is to overcome the inconveniences ofthe known apparatuses.

This object is achieved by an apparatus for checking the machiningprocess of a machine tool according to claim 1 and an associatedchecking method according to claim 10.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described in more detail with reference to theenclosed sheets of drawings, given by way of non-limiting example,wherein:

FIG. 1 is a schematic cross-sectional view of a grinding machine in thecourse of the machining of a mechanical piece and an apparatus accordingto the present invention;

FIG. 2 is a plan view of the grinding machine and the apparatus of FIG.1;

FIG. 3 a is an enlarged scale view of a detail of the grinding machineshown in FIG. 2 in the course of a first machining phase of the piece;

FIG. 3 b is an enlarged scale view of a detail of the grinding machineshown in FIG. 2 in the course of a second machining phase of the piece;

FIG. 4 a shows the trend of the signal output by the acoustic sensor inthe course of the two machining phases shown in FIGS. 3 a and 3 b,performed in sequence;

FIG. 4 b shows the trend of the signal output by the force-detectingsensor in the course of the two machining phases shown in FIGS. 3 a and3 b, performed in sequence;

FIG. 5 a shows the trend of the signal output by the acoustic sensor inthe course of two machinings in which the grinding wheel has differentcutting capacity;

FIG. 5 b shows the trend of the signal output by the force-detectingsensor in the course of two machinings in which the grinding wheel hasdifferent cutting capacity;

FIG. 6 a shows the trend of the signal output by the acoustic sensor inthe course of two machinings performed at different grinding wheelapproach speeds;

FIG. 6 b shows the trend of the signal output by the force-detectingsensor in the course of two machinings performed at different grindingwheel approach speeds;

FIG. 7 is a flow chart showing the operation of an apparatus accordingto the present invention, and

FIG. 8 is a block diagram showing the logic inter-connections of thevarious elements that form the apparatus according to a preferredembodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIGS. 1 and 2 show, in an extremely simplified form, a machine tool, inparticular a computer numerical control (“CNC”) grinding machine 1,including a base 2, a tool-carrier slide 3, more specifically a grindingwheel-carrier coupled to a slide 4, movable with respect to the base 2along an axis Y and coupled to a slide 5 movable along an axis X withrespect to the base 2, in such a way that the tool-carrier slide 3 candisplace with respect to the base 2 in the plane XY. A spindle 7 ismounted on the tool-carrier slide 3 and coupled, by means of a flange11, to a grinding wheel 9. The spindle 7 is coupled to a motor, notshown in the figures, and enables the grinding wheel 9 to rotate about alongitudinal axis M.

The grinding machine 1 is utilized for machining a mechanical piece 13,with rotational symmetry, for example a shaft, supported, arranged andput into rotation about a longitudinal axis P, parallel to thelongitudinal axis M, by means of a support and reference system of theknown type consisting, for example, of a live center 15 and a deadcenter 17.

An apparatus for checking the machining process of the grinding machine1 includes a first sensor, i.e. a force-detecting sensor 21, of a knowntype, with a first portion 22, coupled to the tool-carrier slide 3, anda second portion 25, coupled to the slide 4, a second sensor, morespecifically an acoustic sensor (or AE sensor) 19, also of the knowntype, coupled to flange 11, and a processing and control unit 23electrically connected, in a way that has not been shown in the figures,to the sensors 19 and 21 and to a numeric control 24 of the grindingmachine 1. The force-detecting sensor 21 detects deformations of thetool-carrier slide 3, with respect to the slide 4, in the directionindicated by arrow F in FIG. 1, caused by the force applied by grindingwheel 9 on the piece 13 and provides a first output signal SF. Theacoustic sensor 19 detects the acoustic signals generated by contactbetween the surfaces of the grinding wheel 9 and of the piece 13 in thecourse of the machining and provides a second output signal SA.

FIGS. 3 a and 3 b show two subsequent phases of the machining of thepiece 13, in which the extension of the contact surface between grindingwheel 9 and piece 13 differs. In the condition shown in FIG. 3 a, thereis the maximum amount of contact surface between grinding wheel 9 andpiece 13, whereas in the condition shown in FIG. 3 b just part of thegrinding wheel 9 is in contact with piece 13.

FIGS. 4 a and 4 b show, in a time sequence, the trends of signal SAoutput by the acoustic sensor 19 and of the signal SF output by theforce-detecting sensor 21 respectively, at the two subsequent machiningphases shown in FIGS. 3 a and 3 b.

The instants of time tia, tfa, tib, tfb indicate the instant of start(tia, tib) and the instant of end (tfa, tfb) of contact between grindingwheel 9 and piece 13 in the machining phases shown in FIGS. 3 a and 3 b.

As shown in FIG. 4 a, the signal SA output by the acoustic sensor 19 hasan amplitude that very rapidly varies both as a consequence of grindingwheel 9 and piece 13 coming in touch with each other (instants tia, tib)and as a consequence of the grinding wheel 9 detaching from the piece 13(instants tfa, tfb), but does not substantially depend on the extensionof the surface of contact between grinding wheel 9 and piece 13.

In fact, the time frame in which the signal assumes a high logic valuedepends on the contact time between grinding wheel 9 and piece 13 (timelags tia–tfa, tib–tfb), but its amplitude does not undergo significantmodifications in the course of the two different machining phasesillustrated in FIGS. 3 a and 3 b in which there is a different extensionof contact surface between grinding wheel 9 and piece 13.

On the contrary, the signal SF output by the force-detecting sensorslowly varies after grinding wheel 9 and piece 13 touch each other(instants tia, tib) and mutually detach (instants tfa, tfb), while itsamplitude directly depends on the extension of the contact surfacebetween grinding wheel 9 and piece 13 and more specifically, the largerthe contact surface, the greater the amplitude of signal SF.

FIGS. 5 a and 5 b show the output signal SA of the acoustic sensor 19and the output signal SF of the force-detecting sensor 21 in the courseof two different machinings, that start, respectively, at instants ti1,ti2 and end, respectively, at instants tf1 and tf2, in which grindingwheel 9 has different cutting capacity. The instants ti1 and tf1represent the instant of start and the instant of end of contact betweengrinding wheel 9 and piece 13 under a condition in which grinding wheel9 has low cutting capacity. The instants ti2 and tf2 represent theinstant of start and the instant of end of contact between grindingwheel 9 and piece 13 under a condition in which, at identical conditionsas those of the previous case (for example with regard to the extensionof the contact surface between grinding wheel and piece), the grindingwheel has high cutting capacity (for example after it has undergonedressing).

As shown in the figures, the output signal SA of the acoustic sensor 19does not vary in a significant way if the grinding wheel 9 is more orless sharp, but its trend just depends on the contact time betweengrinding wheel 9 and piece 13. On the contrary, the output signal SF ofthe force-detecting sensor 21 slowly increases after contact occursbetween the grinding wheel and the piece, slowly decreases when thegrinding wheel detaches from the piece, and has an amplitude thatdepends on the cutting capacity of the grinding wheel 9, morespecifically the amplitude of the signal SF increases as the cuttingcapacity of the grinding wheel 9 decreases.

FIGS. 6 a and 6 b show the output signal of the acoustic sensor 19 andthe output signal of the force-detecting sensor 21 in the course of twodifferent machinings, that start, respectively, at instants ti3 and ti4and end, respectively, at instants tf3 and tf4, and that differ as faras the approach speed of the grinding wheel 9 is concerned. The instantsti4 and tf4 are the instant of start and the instant of end of contactbetween grinding wheel 9 and piece 13 under a condition in which theapproach speed is relatively low whereas the instants ti3 and tf3 arethe instant of start and the instant of end of contact between grindingwheel 9 and piece 13 under a condition in which the grinding wheel 9 hasa faster approach speed.

From the figures it is possible to realize that the output signal SA ofthe acoustic sensor 19 does not substantially vary if the approach speedof the grinding wheel 9 varies, but its trend only depends on the amountof time in which there is contact between grinding wheel 9 and piece 13(time intervals ti3–tf3, ti4–tf4). On the contrary, the output signal SFof the force-detecting sensor 21 slowly increases after contact occursbetween grinding wheel 9 and piece 13 (instants ti3, ti4), slowlydecreases at the end of contact between grinding wheel 9 and piece 13and has an amplitude that directly depends on the approach speed of thegrinding wheel 9. Higher approach speeds cause signals SF with greateramplitudes.

In summary, the output signal SA of the acoustic sensor 19 rapidlyincreases when grinding wheel 9 contacts the piece 13 and rapidlydecreases when the grinding wheel displaces away from piece 13, and hasan amplitude that substantially does not depend on the extension of thecontact surface between grinding wheel 9 and piece 13, on the cuttingcapacity and on the approach speed of the grinding wheel 9. An analysisof such a signal provides information on the time frames in which themachining occurs (contact between grinding wheel and piece), regardlessof the machining conditions. The force-detecting sensor 21, instead,generates a signal SF that slowly increases when the grinding wheel 9contacts the piece 13, slowly decreases as they detach from one another,but has an amplitude that depends on the extension of the contactsurface between grinding wheel 9 and piece 13, on the sharpnesscondition of the grinding wheel 9 and on the approach speed of thegrinding wheel 9. Said output signal SF of the force-detecting sensor 21does not enable to accurately define the time intervals in whichmachining occurs but it enables to obtain information on the machiningprocess and more specifically on the contact area between piece 13 andgrinding wheel 9, and on the cutting capacity of the grinding wheel 9and on the approach speed of the grinding wheel 9.

A method according to the invention exploits in a synergetic way theinformation provided by the acoustic sensor 19 and by theforce-detecting sensor 21. More specifically, the output signal SA ofthe acoustic sensor 19, owing to its rapid response features whencontact occurs between the grinding wheel 9 and the piece 13 and to theamplitude substantially uncorrelated to the contact force, is utilizedfor determining the time intervals in which the output signal SF of theforce-detecting sensor 21 provides useful information for checking themachining process, in other words the intervals in which the grindingwheel 9 is actually in contact with the piece 13. The trend of theoutput signal SF of the force-detecting sensor 21 in said time intervalsprovides significant information on the quantity of material that hasbeen removed in the course of the machining (on the basis of theapproach speed of the grinding wheel 9 and on the extension of thecontact surface between grinding wheel 9 and piece 13) and on thesharpness condition of the grinding wheel 9.

The operation of the apparatus according to the invention is nowdescribed with reference to the flow chart in FIG. 7.

In a first phase the numeric control 24 automatically selects, or theoperator can manually select, the type of check to carry out, forexample the wear condition of the grinding wheel 9 (block 40).

Subsequently the processing and control unit 23 checks the logic valueof the output signal SA of the acoustic sensor 19 (block 50). As soon asthe signal SA changes to a high logic value, there is detected aninstant of start, with a substantially negligible delay with respect tothe instant ti in which there occurs the actual contact between grindingwheel 9 and piece 13: the processing and control unit 23 consequentlystarts to monitor the output signal SF of the force-detecting sensor 21(block 60). The processing and control unit 23 checks the logic value ofthe output signal SA (block 70). The monitoring of the output signal SFof the force-detecting sensor 21 continues until the output signal SA ofthe acoustic sensor 19 maintains a high logic value and, consequently,for all the time interval in which the grinding wheel 9 is in contactwith the piece 13 (block 60). When the output signal SA of the acousticsensor 19 changes to low logic value, there is detected an instant ofend, with a substantially negligible delay with respect to the instanttf at which there occurs the actual displacing of the grinding wheel 9away from the piece 13, and the monitoring of the signal SF by theprocessing and control unit 23 ends. The retrieved (and stored) outputsignal SF of the force-detecting sensor 21 undergoes a suitableprocessing for obtaining information relating to the monitored feature,in this case the cutting capacity of the grinding wheel 9, suchprocessing including, for example, the integration of the signal SF in atime interval that substantially corresponds to the interval ti–tf(block 80). Subsequently, the value obtained by the processing iscompared with values memorized and detected in a previous setting or“learning” phase of the apparatus (block 90). If the result of thiscomparison indicates that the monitored feature, in this case the wearof the grinding wheel 9, is within acceptable limits, it is possible tomove on to carry out, if necessary (block 100), another type of checking(block 40). On the contrary, if the monitored feature exceeds theacceptable limits, the numeric control indicates to the operator, forexample by displaying a message, the need to carry out a dressing cycleof the grinding wheel 9, or, once the machining of the piece ends,directly controls, without the operator's attendance, the dressing cycleof the tool (block 110). The procedure ends (block 120) when therequired checkings have been completed.

In a preliminary setting or “learning” phase, that follows theinstallation of the sensors 19 and 21 on the machine 1, the processingand control unit 23 retrieves the output signals of the sensors 19 and21 at different working conditions: different extensions of contactsurface between grinding wheel 9 and piece 13, different wear conditionsof the grinding wheel 9 and different approach speeds of the grindingwheel 9 and processes said signals for obtaining and memorizingreference values to be compared with those that are acquired during thecheckings.

As an alternative to what is described above, after having chosen thetype of check to carry out, the processing and control unit 23 cansimultaneously acquire the output signals SA and SF of both the sensors19 and 21 and, on the basis of the trend of the signal SA, determine theinstant of start and the instant of end of contact, that substantiallydefine the time interval in which machining occurs, and processthereafter, as previously described, the signal SF only at said timeinterval.

The sensors can be arranged on the grinding machine at positions thatdiffer from those herein described, so long as they allow the correctdetecting of the signals. More specifically, the acoustic sensor can becoupled to one of the two centers, or to a grinding wheel balancingdevice, if applied to the grinding machine. Instead, the force-detectingsensor can be arranged between an intermediate support of the piece, ora steady rest, if included, and the base of the machine.

Variants with respect to what is herein described are feasible and morespecifically the force-detecting sensor 21 can be replaced with apower-detecting sensor for detecting the electric power absorbed by themotor of the grinding wheel 9, or with a strain gauge with highsensitivity, for example a strain gauge on a silicon film, fordetermining the deformations of the grinding wheel slide in the courseof the machining and, consequently, the force applied by the grindingwheel 9 on piece 13. The trends of the signals output by thepower-detecting sensor and by the strain gauge are alike that of theabove described signal output by the force-detecting sensor 21 and thusprovide identical information on the machining process. The acousticsensor 19 can be substituted with a sensor of another type that is ableto detect, with appropriate accuracy, the time intervals in which thegrinding wheel 9 is in contact with piece 13. For this purpose there canbe utilized, for example, optical sensors or proximity inductive sensorsthat measure the mutual position between grinding wheel 9 and piece 13.

Furthermore, an apparatus according to the invention can include agreater number of sensors (identified as S1 . . . SN in FIG. 8),identical or of a different type with respect to those herein described,for checking a greater number of processes. The block diagram of FIG. 8shows an apparatus including N sensors S1 . . . SN, connected withassociated conditioning electronics E1 . . . EN, the outputs of whichmeet in a processing unit 27 connected with a unit 29, for determiningthe control procedure, connected with the numeric control 24 and the PLC(Programmable Logic Controller) 31 of the machine 1.

The signals output by the sensors S1 . . . SN are amplified and suitablyprocessed by the conditioning electronics E1 . . . EN before being sentto the processing unit 27. Unit 29 for determining the controlprocedure, on the basis of the requests received by the numeric control24 or by PLC 31, sends signals to the processing unit 27 in order thatit can carry out the necessary processings of the output signal of asingle sensor S1 . . . SN or synergetically utilize the output signalsof a plurality of sensors S1 . . . SN, and sends the results of saidprocessings to the numeric control and/or to the PLC and/or to otherdisplay units as, for example, a personal computer 33.

Many types of checkings can be carried out by an apparatus of this kindand they include automatic checkings of the various machining phases,checkings of the condition of the tools, control of their possiblesharpening, in addition to checking of possible collisions anddiagnosticating for preventive maintenance.

Obviously, an apparatus and a method according to the invention can beutilized for checking the machining process of other types of machinetools, as for example, lathes or milling machines.

1. An apparatus for checking the machining process of a machine tool,the machine tool including a base and a tool-carrier slide, movable withrespect to the base and carrying a tool, the apparatus including: asupport and reference system for supporting and referring a piece to bemachined; a first sensor, adapted for sensing the force occurringbetween the tool and the piece and for emitting a first output signal; asecond sensor, adapted for detecting the contact between the tool andthe piece and for emitting a second output signal indicative of theoccurrence of said contact; and a processing and control unit connectedto the first sensor and to the second sensor and adapted for processingsaid first output signal on the basis of the second output signal andfor providing information on the machining process of the machine tool.2. The apparatus according to claim 1, wherein said second output signalis indicative of a time interval during which there is the contactbetween the tool and the piece, the processing and control unit beingadapted to define said time interval on the basis of the second outputsignal and process said first output signal at said time interval. 3.The apparatus according to claim 2, wherein said second sensor is anacoustic sensor.
 4. The apparatus according to claim 3, wherein saidtool is a grinding wheel coupled to said tool-carrier slide and able torotate with respect to it, said acoustic sensor being coupled to saidgrinding wheel.
 5. The apparatus according to claim 1, wherein saidsecond sensor is an acoustic sensor.
 6. The apparatus according to claim5, wherein said tool is a grinding wheel coupled to said tool-carrierslide and able to rotate with respect to it, said acoustic sensor beingcoupled to said grinding wheel.
 7. The apparatus according to claim 1,wherein said first sensor is a force-detecting sensor.
 8. The apparatusaccording to claim 7, wherein said tool-carrier slide is coupled to aslide, movable along a transverse direction, said force-detecting sensorbeing connected to said tool-carrier slide and to said slide.
 9. Theapparatus according to claim 1, wherein said first sensor is apower-detecting sensor.
 10. The apparatus according to claim 1, whereinsaid first sensor is a strain gauge.
 11. The apparatus according toclaim 1, wherein said machine tool includes a numeric control connectedto said processing and control unit.
 12. A method for checking themachining process of a machine tool, the machine tool including a baseand a tool-carrier slide, movable with respect to the base and carryinga tool, by means of an apparatus, said apparatus comprising a supportand reference system for supporting and referring a piece to bemachined, a first sensor adapted for sensing the force occurring betweenthe tool and the piece and for emitting a first output signal, a secondsensor, adapted for detecting contact between the tool and the piece andfor emitting a second output signal having a logic value indicative ofthe occurrence of said contact, and a processing and control unitconnected to the first and to the second sensors and adapted forprocessing said first output signal on the basis of the second outputsignal, the method including the following steps: checking the logicvalue of the second output signal and detecting an instant of start,substantially corresponding to the instant at which there occurs thecontact between the tool and the piece; monitoring, from said instant ofstart, the first output signal; checking the logic value of the secondoutput signal, and detecting and instant of end, substantiallycorresponding to the instant at which there occurs detachment betweentool and piece; ending the monitoring of the first output signal at saidinstant of end; processing said first output signal; performing acomparison between the results of said processing and reference values;and deciding, on the basis of the result of said comparison, thenecessary operations to be carried out.
 13. The method according toclaim 12, wherein the results of the processing of the first outputsignal are indicative of the quantity of material removed in the courseof the machining of the piece.
 14. The method according to claim 13, forthe checking of the machining process of a grinding machine.
 15. Themethod according to claim 12, wherein the results of the processing ofthe first output signal are indicative of the sharpness condition of thetool.
 16. The method according to claim 15, for the checking of themachining process of a grinding machine.
 17. An apparatus for checkingthe machining process of a machine tool, the apparatus comprising: asupport and reference system for a piece to be machined; a first sensor,adapted for sensing force between a tool of the machine tool and thepiece, and for emitting a first output signal; a second sensor, adaptedfor detecting contact between the tool and the piece, and for emitting asecond output signal; and a processing and control unit, incommunication with the first sensor and the second sensor, and adaptedfor processing said first output signal on the basis of the secondoutput signal.
 18. A method for checking the machining process of amachine tool machining a piece by means of a tool, the methodcomprising: providing a first sensor for sensing force between the tooland the piece, the first sensor emitting a first output signal;providing a second sensor for sensing contact between the tool and thepiece, the second sensor emitting a second output signal; providing aprocessing and control unit for receiving the first and second outputsignals; detecting contact between the tool and the piece based on thesecond output signal; monitoring the first output signal after detectingthe contact; detecting detachment between the tool and the piece basedon the second output signal; ending the monitoring of the first outputsignal after detecting the detachment; and comparing results of themonitoring with reference values.